In many automated industrial processes and climate control systems, it is often necessary to sense and control the ambient humidity level. Capacitive relative humidity sensors are widely used in various areas of applications such as instrumentation, automated systems and climatology, to monitor and detect humidity levels.
Humidity sensors utilize changes in the physical and electrical properties of a sensitive material (e.g., dielectric polymer) when exposed to atmospheric humidity conditions of a surrounding area. In principle, with a capacitive type organic polymer film humidity sensor, the absorbed water vapor occupies free spaces between polymer molecules, and the dielectric permittivity of the polymer is linearly changed proportionally to the amount of absorbed water. Capacitive type humidity sensors thus provide a measure of the humidity based on the amount of absorption or desorption of water molecules and the resultant variation of the dielectric constant of the sensing material, and thus changes in its capacitance. Thus, humidity changes are directly detected by measuring the changes of the capacitance of the sensing material.
In general, the typical configuration of a capacitance sensor is either a sandwich structure with two electrode surfaces on each side, or an interdigitated structure with comb electros and the sensing material (e.g., dielectric polymer) deposited inbetween.
Various attempts have been made to integrate humidity and other chemical capacitance sensors into CMOS structures. For example, capacitive humidity sensors have been fabricated by forming a bottom electrode, depositing a chemical-sensitive polymer layer (e.g., polyimide), and then patterning a set of top electrodes (e.g., conductors). However, this structure is difficult to integrate with testing electronics. In addition, placing the sensitive material layer between two metal layers requires significant processing beyond conventional CMOS.
Another attempt involved coating interdigitated metal electrodes with a chemical-sensitive polymer. While this approach eliminated the necessity of having metal above and below the sensitive layer, a large, parallel capacitance was created through the substrate under the electrodes.
Another technique to produce an integrated chemical capacitance sensor into a CMOS structure involved selectively etching the dielectric of the CMOS to expose a core metal layer (electrically connected in series between two other metal adhesion layers). The exposed core metal layer is then etched to form a cavity between the two metal adhesion layers. The cavity is then filled with an environmental-sensitive dielectric material that is capable of selectively absorbing the chemical to be sensed. For humidity applications, polymers such as polyimide, polymethylmethacrylate (PMMA), poly(ethylene terephthalate (PET), polysulfone (PSF), cellulose acetate butyrate (CAB) and polyethynyl fluorenol (PEFI), are used. The metal adhesion layers then act as the top and bottom electrodes of the environmental-sensitive capacitor. However, this method requires the modification of the CMOS process flow to accommodate the incorporation of humidity sensor.